Downflow mixers of the type disclosed in U.K. Pat. No. 1,428,349 have heretofore been made by the assignee of the present application for mixing materials in an open surface body of liquid. In general, such prior downflow mixers included an axial flow propeller disposed in an upright tubular pump casing having its upper intake end disposed below the surface of the body of liquid and driven by a motor supported above the surface of the body of liquid. The propeller is spaced a substantial distance below the motor and, in the mixer shown in the above patent, the propeller was mounted on a separate propeller shaft that was rotatably supported in radial and axial thrust bearings and connected by a flexible coupling to the motor shaft. At least the lower propeller shaft bearing had to be located below the liquid level to support the propeller shaft adjacent the propeller and had to run submerged in the liquid being pumped. Because of the practical difficulties in sealing and lubricating an oil lubricated bearing when it is submerged in water, the lower bearing was made of the water lubricated type. However, such water lubricated bearings will fail rapidly if the supply of water to the bearing is shut off. On the other hand, the liquid being pumped in the reservoir or pond commonly contains foreign material which is abrasive in nature and which causes bearing and shaft wear and eventual bearing failure. Further, even when the propeller shaft in the prior mixer was firmly supported by the lower submerged bearing, very high levels of vibration were encountered in the mixer during operation. This vibration was sometimes so high as to cause fracture of parts of the downflow mixer due to metal fatigue in a relatively short time, sometimes after only a few weeks or months of operation.